The maximum number of binary trees that can be formed with three unlabeled nodes is:

 A 1 B 5 C 4 D 3

all possible unlabeled binary trees

             O
/     \
O        O
(i)

O
/
O
/
O
(ii)

O
/
O
\
O
(iii)

O
\
O
\
O
(iv)

O
\
O
/
O
(v)